The invention relates most particularly to the batch materials needed to manufacture glasses having a significant content of alkali metals, especially sodium, for example glasses of the silica-soda-lime type used to make flat glass. The batch material currently used most often for providing sodium or potassium is sodium carbonate Na2CO3 or potassium carbonate K2CO3, which choice is not without its drawbacks since, on the one hand, this compound provides only sodium as constituent element of the glass, the entire carbonate part decomposing in the form of evolution of CO2 during melting. On the other hand, this is an expensive batch material, compared with the others, since it is a synthetic product obtained by the Solvay process from sodium chloride and lime, which process involves a number of manufacturing steps and is quite expensive in terms of energy.
This is the reason why it has been proposed to use as sodium source not a carbonate but a silicate, possibly in the form of a mixed alkali metal (Na)/alkaline-earth metal (Ca) silicate prepared beforehand. The use of this type of intermediate product has the advantage of jointly providing several of the constituents of the glass, of eliminating the decarbonatization phase and of reducing CO2 emissions from the melting furnace. It also makes it possible to speed up the melting of the batch materials in their entirety and of favoring their homogenization during melting as indicated, for example, in the patents FR-1 211 098 and FR-1 469 109. However, this approach poses the problem of the manufacture of this silicate.
A first method of synthesis was described in the patent WO-00/46161: this involves the conversion of a halide, for example NaCl, and silica into a silicate at high temperature, the heat supply being provided by submerged burners.
Combustion by submerged burners is already known, for example from the patents U.S. Pat. Nos. 3,627,504, 3,260,587 or 4,539,034, for melting vitrifiable materials to make glass. To use this technology in a context different from the synthesis of silicates, and therefore upstream of the actual glass manufacture, indeed offers many advantages: this method of combustion causes, within the materials undergoing reaction, strong turbulence and vigorous convection motion around the gas jets or flames from the submerged burners. This promotes very effective stirring of the reactants. Furthermore, submerged burners provide the heat directly at the point where it is needed, into the mass of the products undergoing reaction. It is also an environmentally friendly method of combustion.
For further details about the various reactions involved, reference may be made to the aforementioned patent WO-00/46161.
Direct conversion of NaCl and silica carried out in this way is therefore very attractive for more than one reason. However, this direct conversion has proved to be ill suited to implementation on a large scale.
The object of the invention is therefore to develop another type of silicate manufacture, which can retain the advantages of the technique described above, while being easier to use on an industrial scale. Secondarily, it will be attempted to make this novel type of manufacture as environmentally friendly as possible and to take into account/utilize, to the best, all the reaction products involved other than the silicates, the manufacture of the silicates remaining the primary objective of the present invention.